Deep space communications are far more challenging than communications with Earth-orbiting satellites because the distances are so staggering. Signals must travel millions or billions of kilometers between Earth and the spacecraft. At the same time, the spacecraft's communications equipment must be compact and lightweight to allow for a larger scientific payload. Most spacecraft communicate at a very low power - up to 20 times less than the power required for a digital watch.

To hear the whisper of a signal at such great distances, receiving antennas on Earth must be very large, equipped with extremely sensitive receivers and protected from interference. Imagine a person shouting loudly to a companion a block away. If the street is quiet, the person can be understood. But what if the two pals are in Los Angeles and
San Francisco - 700 kilometers (435 miles) apart? That's exactly the kind of conditions the Deep Space Network operates under. The Earth-orbiting satellites are the equivalent of two friends shouting a block apart; while the deep space probes a scattered through the solar system - from 2001 Mars Odyssey at Mars to Voyager 1 at the very edge of our solar system.

The Deep Space Network listens to low-power spacecraft signals with huge antenna dishes that are precisely shaped and pointed at the target with pinpoint accuracy. In a sense, they are giant ears. The network's largest - the 70-meter (230-foot) diameter antenna - stands as tall as a 9-story building.

They also serve as giant megaphones so mission control can talk to the spacecraft. The network's antennas use high power transmitters to broadcast commands to spacecraft, allowing mission controllers to activate computers and instruments and make course corrections.

A Global Network

As if that weren't hard enough, the Deep Space Network must accomplish this while anchored the the surface of a rapidly spinning planet. Imagine you are standing on Callisto, one of Jupiter's moons, looking back towards Earth with a powerful telescope. First, you'd see the United States. A few hours later - as the Earth rotates on its axis - you'd be looking at Australia and still later the European continent would swing into view.

To compensate for this rotation, the Deep Space Network maintains clusters of antennas three locations - in California's Mojave Desert, near Madrid, Spain and outside of Canberra, Australia. The spacecraft signals are received at one site; as Earth turns, the spacecraft "sets" at that site - just like the Sun sets every night - and the next site picks up the signal, then the third and then back to the first. Think of it as a relay race with one runner handing off the baton to the next, who hands it off to a third. This configuration allows NASA to maintain 24-hour contact with distant spacecraft.

The DSN was created in 1958 to track America's first satellite -- Explorer 1 (right).

The first station of what would become the Deep Space Network (above) was located at Goldstone and was named after Pioneers 3 and 4, the first spacecraft with which it communicated. This antenna is now a national monument.